Recent advances in the study of membrane ATPases from gastric mucosa have given rise to a model in which an exchange of H ion for K ion is driven by hydrolysis of ATP at the surface of the oxyntic cell. The proposed study is designed to test this model by examining the transfer of phosphate in the isolated, functioning gastric mucosa. To accomplish this, an important part of the study will be to maximize uptake of labelled phosphate (32 pi) to provide optimal levels of labelled intracellular phosphate. Rate of incorporation of phosphate into ATP can then be followed in the resting and secreting states, as well as subsequent transfer of phosphate to ADP and creatine phosphate. Labelled phosphate is also incorporated into perchloric acid-insoluble fractions of gastric mucosa, and the nature of the labelled material will be examined closely. An increased rate of labelling in the secreting state, compared to resting, will be of considerable significance: it could reveal more rapid phosphorylation of a membrane ATPase, or of a membrane protein phosphorylated by way of a cyclic AMP-dependent protein kinase, or of both kinds of protein.